In U.S. patent application Ser. No. 09/484,184 filed Jan. 18, 2000 (incorporated herein by reference), a battery separator made by a “collapsed bubble” technique is disclosed. “Collapsed bubble” technique refers to a variation of the known blown film extrusion technique for making films. In blown film extrusion, polymer is extruded through an annular die to form a “bubble,” the bubble is cooled and collapsed, but the collapsed bubble does not permanently adhere to itself. See: Berins, M. L., Ed., Plastics Engineering Handbook, 5th Edition, Chapman & Hall, New York, N.Y. (1991) pp. 102-105, incorporated herein by reference. In the “collapsed bubble” technique, when the bubble is collapsed onto itself, it is designed to permanently adhere to itself. In the forementioned application, the extruded polymer is a co-extruded polymer, i.e. two distinct polymers forming two distinct layers.
Celgard® 2402 and 2502 flat sheet membranes are pre-existing products made by a “collapsed bubble” technique. The relevant properties of these membranes are set out in Table 2 below. Celgard® 2400, 2500, and 849 flat sheet membranes are pre-existing products made by a blown film extrusion technique. The relevant properties of these membranes are set out in Tables 1 and 2. Celgard® 2400 and 2500 are related to Celgard® 2402 and 2502 in that the latter two products are collapsed bubble variants of the former products and are twice as thick as the former two products.
In U.S. Pat. No. 5,565,281, there is disclosed a bilayer battery separator, one layer has strength capabilities and the other layer has shutdown capabilities. The two independently made layers are bonded together.
In U.S. Pat. No. 5,667,911, a cross-plied battery separator is disclosed. In this separator, uniaxially oriented plies are bonded together so that they are angularly biased (i.e., an angle of between 20° and 90° between the uniaxial orientation of each ply).
In U.S. Pat. No. 5,922,492, a composite battery separator is disclosed. This separator joins together to different materials, a microporous membrane and a nonwoven fabric are joined by lamination.
In U.S. Pat. No. 5,952,120, a trilayer battery separator having two strength layers sandwiching a shutdown layer is disclosed.